Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

29
Spectroscopy Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

description

The IR Region These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules. type of bonds are present some structural information

Transcript of Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Page 1: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Spectroscopy

Infrared Spectroscopy (IR)Fourier Transform Infrared

(FTIR)

Page 2: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

The IR Region

• These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.

– type of bonds are present

– some structural information

Page 3: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 3

The IR Region• Infrared radiation

λ = 2.5 to 17 μm

ν = 4000 to 600 cm-1

• Units are wave numbers, or cm-1, the reciprocal of the wavelength in centimeters. Wave numbers are proportional to frequency and energy.

Page 4: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Energy Levels: Basic Ideas

Page 5: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 5

Stretching Frequencies

• Frequency decreases with increasing atomic weight.

• Frequency increases with increasing bond energy.

Page 6: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 6

Vibrational ModesNonlinear molecule with n atoms usually has 3n -

6 fundamental vibrational modes.

Page 7: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 7

Fingerprint of Molecule

• Whole-molecule vibrations and bending vibrations are also quantitized.

• No two molecules will give exactly the same IR spectrum (except enantiomers).

• Simple stretching: 1600-3500 cm-1.• Complex vibrations: 600-1400 cm-1, called the

“fingerprint region.” =>

Page 8: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 8

IR-Active and Inactive

• A polar bond is usually IR-active.• A non-polar bond in a symmetrical

molecule will absorb weakly or not at all.

Page 9: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 9

Carbon-Carbon Bond Stretching

• Stronger bonds absorb at higher frequencies:– C-C 1200 cm-1

– C=C 1660 cm-1

– CC 2200 cm-1 (weak or absent if internal)• Conjugation lowers the frequency:

– isolated C=C 1640-1680 cm-1

– conjugated C=C 1620-1640 cm-1

– aromatic C=C approx. 1600 cm-1 =>

Page 10: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 10

Carbon-Hydrogen Stretching

• Bonds with more s character absorb at a higher frequency.– sp3 C-H, just below 3000 cm-1 (to the right)– sp2 C-H, just above 3000 cm-1 (to the left)– sp C-H, at 3300 cm-1

=>

Page 11: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 11

An Infrared Spectrometer

=>

Page 12: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

FTIR (Fourier Transform Infrared)

• FTIR spectrometer obtains an infrared spectra by first collecting an interferogram of a sample signal using an interferometer,

• FTIR then performs a Fourier Transform on the interferogram to obtain the spectrum. – Fourier transform defines a relationship between

a signal in time domain and its representation in frequency domain.

Page 13: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

FTIR (Fourier Transform Infrared)

• An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them.

• Michelson Interferometer.– A mirror moves at a fixed rate. Its position is determined accurately by

counting the interference fringes of a collocated Helium-Neon laser. – The interferometer splits a beam of radiation into two paths having

different lengths, and then recombines them. – A detector measures the intensity variations of the exit beam as a

function of path difference.

Page 14: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Schematic of Michelson Interferometer

Page 15: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 15

Summary of IR Absorptions

=>=>

Page 16: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 16

An Alkane IR Spectrum

=>

Page 17: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 17

An Alkene IR Spectrum

=>

Page 18: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 18

An Alkyne IR Spectrum

=>

Page 19: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 19

O-H and N-H Stretching

• Both of these occur around 3300 cm-1, but they look different.– Alcohol O-H, broad with rounded tip.– Secondary amine (R2NH), broad with one sharp

spike.– Primary amine (RNH2), broad with two sharp

spikes.– No signal for a tertiary amine (R3N) =>

Page 20: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 20

An Alcohol IR Spectrum

=>

Page 21: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 21

An Amine IR Spectrum

=>

Page 22: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 22

Carbonyl Stretching

• The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1.

• Usually, it’s the strongest IR signal.• Carboxylic acids will have O-H also.• Aldehydes have two C-H signals around

2700 and 2800 cm-1. =>

Page 23: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 23

A Ketone IR Spectrum

=>

Page 24: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 24

An Aldehyde IR Spectrum

=>

Page 25: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 25

O-H Stretch of a Carboxylic Acid

This O-H absorbs broadly, 2500-3500 cm-1, due to strong hydrogen bonding.

=>

Page 26: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 26

Variations in C=O Absorption

• Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1.

• The C=O group of an amide absorbs at an even lower frequency, 1640-1680 cm-1.

• The C=O of an ester absorbs at a higher frequency, ~1730-1740 cm-1.

• Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency. =>

Page 27: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 27

An Amide IR Spectrum

=>

Page 28: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 28

Carbon - Nitrogen Stretching

• C - N absorbs around 1200 cm-1.• C = N absorbs around 1660 cm-1 and is

much stronger than the C = C absorption in the same region.

• C N absorbs strongly just above 2200 cm-

1. The alkyne C C signal is much weaker and is just below 2200 cm-1 . =>

Page 29: Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

Chapter 12 29

A Nitrile IR Spectrum

=>